The present invention relates to a battery monitoring device and more particularly to a monitor which examines the slope of the charging current of a nickel-cadmium battery to determine if the battery is going into a thermal runaway condition.
Nickel-cadmium batteries, comprising a plurality of nickel-cadmium voltaic cells connected in series, possess a number of highly desirable characteristics that include a relatively high capacity to weight ratio, a relatively flat volatage to percent of discharge curve, good performance at low temperatures, the capability of delivering high amperage currents for engine starting and similar purposes, and a relatively high recharge recycling capacity. Moreover, the chemistry of the nickel-cadmium system is such that nickel-cadmium cells may be hermetically sealed which permits operation in any position and in inaccessible locations. All of these characteristics have made nickel-cadmium batteries particularly well suited for use in aircraft and similar environments.
Aircraft electric systems normally include a rechargeable storage battery, such as a nickel-cadmium battery, and a constant potential battery charging apparatus connected to the battery for restoring and maintaining its charge. However, when a nickel-cadmium battery is connected to a constant potential source of charging current the battery is susceptible to a malfunction known as "thermal runaway" which is a condition that can destroy the battery if undetected and unchecked. At some point during charge, the temperature increases, which is reflected by a decrease in the internal resistance of the battery. On a constant potential charge the current through the battery must increase. As the current increases, however, more heating occurs which further increases the temperature and is followed by a continued decrease in internal resistance and current increase and thus the thermal runaway cycle has begun.
The problem of thermal runaway represents a potential hazard to the aircraft battery, airframe, and flight crew in aircraft utilizing nickel-cadmium batteries for engine starts. The Federal Aviation Administration (FAA) recognized this problem and issued an Airworthiness Directive Revision requiring all aircraft having a primary electrical system including a nickel-cadmium battery to have some type of a sensing and warning device. The system was to be either (1) a battery charging rate control system, (2) a battery temperature sensing and over-temperature warning system, or (3) a battery failure sensing and warning system.
As a result of the FAA directive, numerous endeavors to develop a satisfactory detection and warning system have been attempted. Since the directive requiring the devices has been issued within the last three years, many devices are still in the development stage and some devices are available with only limited application.
One method of circumventing the thermal runaway problem incorporates the use of a battery charging rate control system. Although there are a number of charger systems used on some aircraft, they are not without their drawbacks. Charging control systems will ultimately protect the airframe and the battery from excessive damage, but the battery and airframe must be modified because of the size and weight of the charger. Any modification of the airframe and battery is an expensive and undesirable solution to the thermal runaway problem. Other disadvantages to the charging rate control system include excessive cost and considerably longer development time.
Another mode of treating the thermal runaway problem is by temperature sensing which is the easiest to implement, but is also the least effective because it deals with the effects instead of the cause. With this method, a temperature sensing device is installed internally in the battery between individual cells or on the intercell link. However, this requires a costly modification of the battery. Also, temperature sensing devices detect the results of the thermal runaway instead of the cause, which is overcharging. By the time the battery temperature has risen to a level where thermal runaway and high operating temperature can be discriminated between, permanent damage to the battery may have occured. This type of warning system cannot be universally adapted to all aircraft because the operating temperature of batteries varies from one aircraft to another and also the operating temperature varies from battery to battery. Where to locate a temperature sensor in a battery is another problem with temperature sensing. Thermal runaway begins in a localized area of the battery and unless the temperature sensor is positioned near the local area, serious damage to the battery may result before the condition is detected.
In U.S. Pat. No. 3,940,679, which issued Feb. 24, 1976, to Rowland Brandwein and Mohan Gupta, there is disclosed a nickel-cadmium battery monitor which continuously monitors three battery parameters, namely, the battery temperature, the battery voltage and the rate of change in the battery charging current. Sensors are provided for measuring the temperature of the battery, for measuring the voltage at the terminals of the battery and for measuring the magnitude of the current being discharged by and being charged to the battery, each of said sensors generating a sensor signal voltage that varies directly with the magnitude of the parameter being measured. A temperature sensor signal voltage conditioning and calibrating means is electrically connected to the temperature sensor, and a voltage sensor signal conditioning and calibrating means is electrically connected to the voltage sensor. Each of the conditioning and calibrating means transforms the sensor signal voltage from the sensor connected thereto to a conditioned analog signal voltage, the magnitude of the conditioned signal voltage being in the same relative proportion to a predetermined reference voltage as the actual magnitude of the battery parameter being measured is to a predetermined reference magnitude of this parameter. Continuous voltage gradient to incremental voltage gradient converter means are electrically connected to each of said signal voltage conditioning and calibrating means. The continuous to incremental voltage gradient converter means divides the continuous voltage gradient transmitted by the signal voltage conditioning and calibrating means into a predetermined number of incrementally increasing analog signal voltage increments, whereby the actual signal voltage from each of said signal voltage conditioning and calibrating means is converted to an analog signal voltage increment the magnitude of which corresponds approximately to the actual magnitude of the battery temperature and battery voltage as measured. Analog signal voltage increment indicator means are electrically connected to each of the continuous to incremental voltage gradient converter means, each of said indicator means having a plurality of electrically energized visual display means. Each of the visual display means corresponds to and is indirectly energized by one of the analog signal voltage increments, whereby the indicator means visually indicates the approximate magnitude of the battery temperature and battery voltage as measured.
Also in the above-referenced patent, charge rate sampling means are electrically connected to the battery charge sensor, the sampling means continuously measuring the magnitude of successive samples of the charge current and transmitting an alarm signal to a flash alarm means when the rate of increase of the charge current exceeds a predetermined value. In the preferred embodiment, the charge rate sampling means continuously measures successive samples of the charge current and records a "bit" in a memory element of the sampling means each time the magnitude of the charge current of a given current sample exceeds by a predetermined amount the charge current of the immediately preceding current sample. The sampling means transmits an alarm signal to the flash alarm means when the number of current samples (or "bits") recorded in the memory element during a predetermined period of time exceeds a predetermined number of bits. The flash alarm means is electrically connected to each of the aforesaid continuous to incremental voltage gradient converter means and to the aforesaid charge rate sampling means. The flash alarm means transmits a flash alarm signal to the analog-to-digital converter means when the temperature of the battery exceeds a predetermined value, or when the voltage of the battery falls below or exceeds a predetermined value, or when an alarm signal is received from the charge rate sampling means.